[REFACTOR] Unify the representation of the type field in targeting strategies

[ncontinanza]

🚧 Refactor

Check the first and last comment left in: lambdaclass/mirra_backend#647

In our current targeting strategies, the type field is a string in some cases and a map in others. It would be better to have type as a string and a separate stat field for the cases in which corresponds (highest and lowest targeting strategies, for instance).

This refactor will help to simplify the implementation of create_mechanics_map/3, where we're currently implementing different behaviors for the string and the map case.

Also, talk with Game Design in order to make this change in the design documents.